Return of Caligula and Yeti were early boss fights, and at that point I wasn't "deep" into knowledge about the mechanics of the gameplay. Return of Caligula was difficult because you have to protect a statue from harm (not that Caligula harms your team at all, but he makes quick work of that statue). Yeti could freeze party members, rendering them temporarily immobile, and if he froze all three members, it's game over.

However, definitely the two most difficult ones, were Lagerfield and Rowan. Against Lagerfield, part of the problem was that you only have one party member. In addition, the guy teleports all over the place and can kill you in one attack. Took me over a dozen attempts to figure out the pattern to his teleportation. Meanwhile, Rowan is understandably difficult as well, being the final boss battle. Most of his difficulty lies in the fact he's got a half dozen soldiers helping him (some of which can inflict poison). And even if you kill the soldiers, or only some of them, they respawn fairly quickly.

I'd have to agree with Lupin3rd, still getting to grips with everything in the game, that Tar Man can be quite tricky. But once you're down with the game,none of the bosses were actually 'hard', I thought. Just have to know how to win.

While not a boss, I remember this one fight giving me fits. It was one of those red-hex optional battles against a level 40-something frag dwellest and 2 low level gunmen. I went there at my first opportunity and was somewhere around level 12 for all the characters.

It was a brutal fight with no worthwhile reward, but I kept retrying until I beat it. I used a whole lot of grenades and first-aid kits considering the big guy could break a bezel in 2 hits. Got a bunch of level-ups though.